1. Field of the Invention
The present invention relates to a recording optical system utilizing a semi-conductor laser as the light source.
2. Description of the Prior Art
Conventionally such optical system is known, for example as a scanning system, to be composed, as shown in FIG. 1, of a laser light source 1 such as a He-Ne laser or an Ar laser, a modulator 2 for modulating the beam from said laser, a deflector 3 such as a rotating polygonal mirror for deflecting thus modulated beam, a scanning lens 4, and a photosensitive medium 5. Such conventional optical system is disadvantageous in that not only the laser but also the modulator is expensive. Also the space of the laser and the modulator in the entire system is not negligible as the laser itself is of a considerably large size and the modulator requires a space of 200-300 mm in the optical path. In contrast to such conventional system, the use of a directly modulatable semi-conductor laser allows one to obtain an inexpensive compact system.
Although the optical system utilizing a semi-conductor laser as the light source was already disclosed for example in RCA Review Vol. 35, September 1974, p. 335-340 and in RCA Review Vol. 36, December 1975, p. 744-758, the laser utilized in these references shows significant variation in the operating mode in response to current variation, which leads to a considerable change of the size of focused spot. Such multi-mode light source is unable to provide a satisfactory image quality in the recording system.
Also in general the semi-conductor laser shows an astigmatism in that the beam waist position in a plane parallel to the junction plane thereof is different from that in a plane perpendicular to said junction plane. For this it is generally difficult to obtain a beam spot of a desired diameter and with a sufficient light intensity on a photosensitive medium, and this fact leads to a limitation in the image resolution.
Though such astigmatism is certainly correctable, the optical system required therefor requires cumbersome adjustment which is not desirable for industrial production.